Abstract Backgrounds Costs of intensive care reach up to 30% of the hospital budget with workforce expenses being substantial. Determining proper nurse—patient ratio is necessary for optimizing patients’ health related outcomes and hospitals’ cost effective functioning. Objectives To evaluate nurses’ workload using Nine Equivalents of Nursing Manpower Use Score and Nursing Activities Score scoring systems while assessing correlation between both scores and the severity of illness measured by Simplified Acute Physiology Score II. Design A Prospective study Settings Cardiac Surgery Intensive Care Unit of the Clinical Hospital Centre Rijeka, Croatia, from October 2014 to February 2015. This Intensive Care Unit has 3 beds that can be expanded upon need. Participants The study included 99 patients treated at this Unit during the study’s period. The scores were obtained by 6 nurses, working in 12 h shifts. Methods Measurements were obtained for each patient 24 h after admission and subsequently twice a day, at the end of the day shift (7 pm) and at the end of the night shift (7 am). The necessary data were obtained from the patient’s medical records. Results Nursing Activities Score showed significantly higher number of nurses are required for one 12 h shift (Z = 3.76, p < 0.001). Higher scores were obtained on day shifts vs. night shifts. (Nursing Manpower Use Score, z = 3.25, p < 0.001 ; Nursing Activities Score, z = 4.16, p < 0.001). When comparing Nursing Activities Score and Nursing Manpower Use Score during the week, we calculated higher required number of nurses on weekdays than on weekends and holidays, (Nursing Manpower Use Score, p < 0.001 ; Nursing Activities Score, p < 0.001). Correlation analysis of Nursing Activities Score and Nursing Manpower Use Score with Simplified Acute Physiology Score II has shown that Nursing Manpower Use Score positively associated with severity of disease, while Nursing Activities Score shows no association. Conclusion Both scores can be used to estimate required number of nurses in 12-h shifts, although Nursing Activities Score seems more suitable for units with prolonged length of stay, while Nursing Manpower Use Score appears better for units with shorter duration of stay (up to four days). Higher workload measured by Nursing Manpower Use Score scale can be predicted with higher Simplified Acute Physiology Score II. However, with low Simplified Acute Physiology Score II scores it cannot be assumed that the nursing workload will also be low. Further research is needed to determine the best tool to asses nursing workload in intensive care units.